Optical scanner for directing electromagnetic radiation to different locations within a scan field

A method of determining an alignment of an optical scanner for directing a laser beam to locations within a scan field in a laser-based powder bed fusion apparatus,

• the laser-based powder bed fusion apparatus comprising:
• a fibre-laser for generating the laser beam;
• the optical scanner for directing the laser beam to a plurality of locations on a powder bed for melting powder;
• a photodetector for detecting light at a laser wavelength of the laser beam;
• and
• a splitter for removing back-reflected laser light travelling backwards in a delivery fibre optic or a beam delivery optic of the fibre-laser and directing the back-reflected laser light to the photodetector, and
• the method comprising:
• locating a reference element within the scan field of the optical scanner;
• controlling the optical scanner to cause the laser beam to be directed to a plurality of different points in the scan field, including at least one point on the reference element;
• detecting the back-reflected laser light with the photodetector;
• determining when the laser beam is directed to a reference position in the scan field given by the reference element from a comparison of an intensity of the back-reflected laser light detected for the different points;
• and
• determining a corresponding demand signal that causes the optical scanner to direct the laser beam to the reference position.